1. Field of the Invention
The present invention relates to a system for detecting deterioration of a catalytic converter which is disposed in an exhaust system of an engine for purifying exhaust gas discharged from engine cylinders.
2. Description of the Prior Art
In a system including oxygen concentration sensors (hereinafter referred to as "O.sub.2 sensors") upstream and downstream of a catalytic converter, it has been known that a period of an output signal of the downstream-side O.sub.2 sensor becomes greater than that of the upstream-side O.sub.2 sensor due to a so-called storage effect of the catalytic converter under a normal condition of the catalytic converter. It has been also known that, when the catalytic converter is deteriorated to lower its storage effect, a period of an output signal of the downstream-side O.sub.2 sensor becomes smaller so as to be substantially the same as that of the upstream-side O.sub.2 sensor, and that an amplitude of the output signal of the downstream-side O.sub.2 sensor becomes greater when compared to that under the normal condition of the catalytic converter.
In consideration of this, there has been proposed a system for detecting deterioration of the catalytic converter as disclosed, such as, in Japanese Unexamined Patent Publication (Kokai) No. 61- 286550 which corresponds to U.S. Pat. No. 4,739,614.
Specifically, in this conventional system, a ratio of a period of an output signal V1 of the upstream-side O.sub.2 sensor to a period of an output signal V2 of the downstream-side O.sub.2 sensor is derived. The system determines that the catalytic converter is deteriorated when this derived ratio becomes greater than a predetermined value or when an amplitude of the output signal V2 of the downstream-side O.sub.2 sensor exceeds a predetermined value.
However, in the foregoing conventional system, when an output characteristic of the upstream-side O.sub.2 sensor is deteriorated to cause a response delay, the period of the output signal V1 of the upstream-side O.sub.2 sensor becomes greater so that the output signal V2 of the downstream-side O.sub.2 sensor is inverted between rich and lean sides with respect to a stoichiometric air-fuel ratio indicative value or a comparison voltage VR2 at the same period as that of the upstream-side O.sub.2 sensor, as shown in FIG. 6(a). As a result, the above-noted ratio between the periods of the output signals of the upstream-side and downstream-side O.sub.2 sensors becomes larger so that the system determines the deterioration of the catalytic converter even when the catalytic converter is working normally.
Further, as shown in FIG. 6(b), when the upstream-side O.sub.2 sensor is deteriorated, an amplitude W2 of an air-fuel ratio correction coefficient FAF which is determined based on the output V1 of the deteriorated upstream-side O.sub.2 sensor becomes greater than an amplitude W1 of an air-fuel ratio correction coefficient FAF.sub.1 which is determined based on the output V1 of the normal upstream-side O.sub.2 sensor as shown in FIG. 5(b). As a result, an air-fuel ratio of an air-fuel mixture which is derived based on the coefficient FAF largely fluctuates so that an amplitude of the output signal V2 of the downstream-side O.sub.2 sensor also increases depending on increment of a fluctuation width of the air-fuel ratio. This means that the amplitude of the output signal V2 of the downstream-side O.sub.2 sensor increases as the period of the output signal V1 of the upstream-side O.sub.2 sensor increases. For this reason, in the conventional system which determines the deterioration of the catalytic converter when the amplitude of the output signal V2 of the downstream-side O.sub.2 sensor exceeds the predetermined value, the increment of the amplitude of the output signal V2 of the downstream-side O.sub.2 sensor due to the deterioration of the upstream-side O.sub.2 sensor is erroneously determined as having been caused by the deterioration of the catalytic converter.
In order to overcome the foregoing disadvantages, there has been proposed a catalytic converter deterioration detecting system of another type as disclosed, such as, in Japanese Unexamined Patent Publication (Kokai) No. 3-134241.
Specifically, in this conventional system, a variation amount during a predetermined time period of an auxiliary air-fuel ratio correction coefficient which is derived based on the output of the downstream-side O.sub.2 sensor, is calculated. The system determines the deterioration of the catalytic converter when this variation amount is below a predetermined value.
However, in this proposed system, the variation amount for the given time period of the auxiliary air-fuel ratio correction coefficient largely changes depending on an updating amount per correction of the auxiliary air-fuel ratio correction coefficient. Further, this variation amount largely differs depending on which of air-fuel ratio feedback control constants is used as the auxiliary air-fuel ratio correction coefficient among a skip mount, an integral mount and a delay time. Still further, these auxiliary air-fuel ratio correction coefficients are set to various different values depending on types of the engines. Accordingly, criteria for determining the deterioration of the catalytic converter should be precisely adapted to respective auxiliary air-fuel ratio correction coefficients. In addition, since the updating amount of the auxiliary air-fuel ratio correction coefficient is not determined in consideration of detecting the deterioration of the catalytic converter, the detection accuracy of the catalytic converter deterioration can not be ensured.